Electronic elements, such as integrated circuits and electronic components, are commonly mounted to substrates such as dies, component or sub-assembly packages, and printed wiring boards.
A typical printed wiring board includes a base layer of an electrically insulating material, such as phenol resin. One side of the board is designed for mounting one or more electronic elements having electrodes, such as conductive pins. Apertures, or through-holes, in the board are designed to receive the conductive pins. The other side of the board houses one or more regions at or near the through-holes, where external terminals, such as solder terminals, may be applied to the conductive pins.
Because it is important to establish reliable electrical connections within and between electronic elements mounted on a substrate, solder terminals for conductive pins have traditionally been strengthened through the use of assembly parts, such as eyelets, or by plating through-holes with copper. Solderability has also been improved, and cost reduced, by filling the through-holes and/or the terminal regions with a conductive paste. Conductive pastes are compounds of conductive metal particles, such as copper, silver, nickel, tin, and lead and its alloys, and binding resins, mixed in predetermined amounts, applied in semi-liquid paste states, then hardened by a process such as thermo-setting.
When such copper-containing conductive pastes are exposed to air, however, a layer of oxidation tends to form on the surface of the conductive paste. The oxidation layer may act as an insulator, compromising solder terminals formed on or near the oxidation layer.
One way to enhance solderability of the conductive paste is to apply a film of silver to the surface of the conductive paste prior to hardening it. If, however, a portion of the silver melts down or migrates during soldering, solder terminals may still suffer from reduced integrity, silver may contaminate the solder bath, and insulation in other areas of the board may be compromised.
There are, therefore, needs for structures and methods for electrically connecting an electronic element and a substrate such as a printed wiring board, which use copper conductive paste, and which have improved solderability and terminal strength, without requiring a silver film to be deposited on the conductive paste.